Many enclosures that contain sensitive instrumentation or equipment must maintain very clean environments in order to operate properly. Examples include: enclosures with sensitive optical surfaces or electronic connections that are sensitive to particulates and gaseous contaminants which can interfere with mechanical or electrical operation; data recording devices, such as computer hard disk drives that are sensitive to particles, organic vapors, moisture, and corrosive vapors; and electronic control boxes such as those used in automobiles that are sensitive to moisture buildup and corrosion as well as contamination from fluids and vapors. Contamination in such enclosures originates from both inside and outside the enclosures. For example, in computer hard drives, damage may result from external contaminates as well as out-gassing from internal components.
One serious contamination-related failure mechanism in computer disk drives is static friction or “stiction.” Stiction is the adhesion of a drive head to a disk while the disk is stopped. Newer high-density disks are more sensitive to contamination-related stiction because they are smoother and only thin layers of lubricants are used. Contaminants on the disk change the surface energy and the adhesive forces between the disk and the head, which causes stiction. Also, vapors that condense in the gap between the head and disk can cause stiction. Further exacerbating these effects, new disk drives have smaller, low energy motors with lower torque.
In addition, disk drives must be protected against a large number of contaminants in the surrounding environment. This is true for drives used in small to medium sized computer systems which may not be used in the typical data processing environment and is especially true in drives that are removable and transportable to any environment, such as disk drives that are used in Personal Computer Memory Card International Association (PCMCIA) Slots, iPod® music devices, and in cell phones.
One successful approach to controlling contamination has been the use of sorbent filters. Sorbent filters must keep the enclosures free of contamination from both internal and external sources. In addition to requirements to provide cleaner environments, filters must be made smaller to fit into small enclosures. An excellent example of space constraints in modern electronic components is in the area of computer disk drives. Today, PCMCIA computer disk drives or drives with 1.8″ (45 mm) disks have gigabytes of storage capacity and are only approximately 5 cm wide and 7.5 cm long. Type 3 PCMCIA drives have a maximum thickness of 10.5 mm, Type 2 drives have a maximum thickness of 5 mm, and Type I drives have a maximum thickness of 3.3 mm. Additionally even smaller 1.0″ (25 mm) and 0.8″ (20 mm) drives for camera and cell phone applications are either on or coming to the market.
A commercially successful sorbent filter is disclosed in U.S. Pat. No. 4,830,643 issued to Sassa et al. This patent teaches a sorbent filter where a powdered adsorbent is encapsulated in an expanded PTFE tube. This tube filter is manufactured by W. L. Gore and Associates, Inc., Elkton, Md., and is commercially available under the trademark GORE SORBER® module. While this apparatus is highly effective, the filter is currently available only in large and medium sizes (e.g., filter volumes down to about 3 cc). In its present form, this filter is incapable of fully addressing growing needs for even smaller and more compact sorbent filters containing a higher sorbent density.
Sorbent filter manufacturers have encountered several obstacles in producing very small tube sorbent filters. As tube size (diameter) decreases, filling the tube with sorbent powder becomes more difficult. This problem is compounded by the common use of larger granular sorbent powders commonly used to avoid ‘dusting’ contamination. It is more difficult to fill the small tube filters without having the powder settle on the external tube surfaces and the seal areas. The powder on the outside of the tube can contaminate the devices near the tube and powder in the seal area can prevent the outer tube from sealing, which may also cause dusting problems through leakage.
Many new applications for sorbent filters require spill proof sorbent materials. However, the use of loose particles in existing filled tube filters, if broken, could spill the loosely packed adsorbent material into the enclosure, damaging the integral components.
Another problem with tubular sorbants is that they require custom automation in production. The rapid development and obsolescence of materials and parts in the disk drive filtration industry makes such custom automation undesirable.
Another sorbent filter commercially available from W. L. Gore & Associates, Inc., called a GORE-TEX® Stand Alone Adsorbent Assembly, consists of a composite sorbent-filled PTFE planar core which is laminated on its top and bottom surfaces with a porous expanded PTFE membrane. This filter fits into slots in an enclosure interior. The sorbent-filled PTFE core can be filled with various sorbent materials selected to adsorb hydrocarbons, moisture, out-gassed plasticizers, corrosives, etc. Although this sorbent assembly provides a low profile compact sorbent assembly, concern has been expressed that the unsealed sides of this device may not provide adequate protection from shedding of sorbent material particles in some applications.
Adsorbent assembly filters manufactured with rotary die cutting equipment and continuous film processes are known. Such equipment is advantageous because it has relatively low set up costs and is highly adaptable. Moreover, continuous film feeding processes greatly reduce part cost.
However, known rotary die cutting equipment imposes significant limitations on adsorbent assembly and adsorbent breather filter design. For example, such equipment is capable of cutting only relative thin adsorbents. This substantially limits total adsorbent capacity for an adsorbent assembly. Thicker adsorbents have also required complex molding of the porous membrane cover to cover and seal the adsorbent within the filter. There are also some other limitations in part designs (such as exposed adhesives), part layers, and the ability to automatically pick and place parts from some rotary processed layouts that can limit the use of these parts. Also there can be limitations on the number of parts per foot that can be made from raw materials that increase per part costs.
Accordingly, there is a need for improved adsorbent assembly and adsorbent breather filters that overcome the foregoing limitations.